Subscription fees on a credit card can be a surprising pain for consumers who don't keep an eagle eye on their statement, adding up to hundreds and even thousands of dollars a year. It's not just the memberships you actively use, either.

Gray-area charges — including fees that kick in after a free trial, auto-renewing services and subscription fees that creep higher over time — won out as consumers' top financial pet peeve in a February MyBankTracker.com survey, with 40 percent of consumers calling them out as a problem. There might also be subscriptions you weren't aware you signed up for, and so-called zombie memberships that live on after an attempt to cancel.

"There's a number of ways in which consumers, to put it not so nicely, can get screwed in this situation," said Linda Sherry, director of national priorities for Consumer-Action.org.

At Truebill, the typical user has 11 recurring charges — mostly services users know about and want, said Truebill founder and chief executive Yahya Mokhtarzada. Netflix is the most commonly found charge, per site data, with Spotify, Amazon Prime, AT&T and GoDaddy domain name hosting rounding out the top five.

Among the 17 percent of site users who cancel a membership, however, the average annual savings is $512. (See chart below for some of the most-canceled charges.)

"As consumers keep racking up more and more subscriptions, it becomes impossible to stay on top of them," Mokhtarzada said.

Food writer Jennifer Perillo found five subscriptions to services she wasn't using, when she went over her statements this spring in preparation for tax filing. Among them, an eFax subscription used once to send camp physicals that couldn't be emailed, and a Weight Watchers membership she thought was long-canceled.

"It feels like it's a whole other job staying on top of these auto-pay charges," she said.

Canceling can be a challenge in itself, one that the bill-tracking sites are happy to help with. Truebill offers cancellations for free. At Trim, most are free, although hard-to-cancel subscriptions requiring certified mail or a phone call cost $6 apiece.

But there's good reason to call on your own first, unless you're pressed for time: Putting money back in your pocket. Customer service reps may be willing to refund recent charges, and may have leeway to cut rates if the reason you're canceling is a service's high price tag.

In under 10 minutes on the phone, Perillo said, she was able to reverse a recent $70 three-month membership charge for Sittercity, which she doesn't use during the school year. The rep also threw in a promo code for when Perillo signs back up again for summer babysitting. With Weight Watchers, Perillo was able to get a refund for three of the four months her subscription lived on after her initial attempt to cancel.

Consumers should also take steps to better monitor their recurring charges. Check the fine print before signing up for a free trial and keep an eye on your statements even after you cancel, Sherry explained.

"If you missed that cancellation deadline, you signed up for contractual terms," she said. "You really, really need to look at these things. There is no free in free trials."

Perillo now sets calendar reminders ahead of the ends of free trials and when regular subscriptions renew, so she can cancel before incurring new bills.

Shift how you set up payments, so that you're sending money instead of allowing a company to pull it from your account, Sherry said. Not only does that make you more aware of recurring charges, but it also nixes the risk of overdraft fees should one of those bills increase.

It also helps to pay with a credit card rather than a debit card.

"The credit card gives you somewhat more staunch dispute rights," she said.